The functional block diagram of FIG. 1 shows a basic form of an RO desalination system of known art without energy recovery. An RO (reverse osmosis) unit 10 is a hydraulic device, “hydraulic” being defined as “operated or effected by the action of water or other fluid of low viscosity”.
In a primary liquid flow, pressure pump 12 receives seawater at its input node “a” at relatively low pressure from preconditioning and filtration apparatus. Pump 12, typically driven by an electric motor 14, develops high pressure at node “b”, the input port of seawater chamber 10A of the RO unit, where the pressurized seawater is forced against an RO membrane 10B, typically polyamide thin-film composite that will not pass sodium or chloride ions.
About 40% of the input liquid traverses membrane 10B to compartment 100 as desalinated water, available to be drawn off as required at outlet “c”. The remaining 60% of the input liquid including extracted residue leaves chamber 10A as a secondary flow of brine from RO brine exit port “d” where it passes through a back-pressure regulating valve 17 to discharge port “e” where the secondary brine flow is discharged to a drainage system as wastewater, typically returned to the sea.
It has long been recognized that there is a substantial amount of energy Ed available in the secondary liquid flow at RO brine exit port “d”, where, compared to 100% pressure Pb and flow rate Rb of the primary flow at input port “b”, the pressure Pd is typically 99% and the flow rate Rd is 60%.
The energy at the RO brine exit port “d” can be calculated from the product of pressure and flow rate (Ed=Pd*Rd): for the foregoing conditions, Ed is found to be 59.4% of Eb. Since the discharge at node “e” is typically at very low pressure, most of the waste energy is dissipated at valve 17.
If this wasted energy could be totally recovered and utilized to reduce the electrical power consumption of the input pump, the net energy recovery of 59.4% would reduce the operating energy cost to 40.6% of the operating energy cost of the basic non-recovery system of FIG. 1. However, practical systems can only approach this limit due to unavoidable machine losses such as friction of bearings and seals, leakage past pistons, seals and valves, turbulence, etc.
FIG. 2 illustrates a system as in FIG. 1 to which energy recovery has been applied by the addition of an energy exchanger 18, connected into the RO brine discharge flow path between nodes “d” and “e” in place of valve 17. Energy recaptured from pressure drop in this flow is fed back to the primary liquid flow side between nodes “a” and “b” as indicated by the arrow.
There have been many different ways suggested and tried for implementing this energy feedback. It can be applied as torque to shaft 16 and/or via an auxiliary pump or equivalent introduced in series and/or parallel with the existing main seawater pump 12 to reduce its load due to pressure and/or flow rate, and thus reduce the electric power consumption of drive motor 14. The efficiency of this energy exchange system is critically important since it directly affects the actual amount of operating cost savings realized. Electric motor efficiency is about 90-95% and pump efficiency ranges from 50 to 90%, typically 80%, so these machines are generally selected for high efficiency.
Due to the cost incentive, energy exchange systems have been the subject of much design research, development and refinement, and with increasing concern about world wide consumer water availability, there are continued ongoing efforts to reduce the cost of desalination through increased efficiency: it is to this end that the present invention is directed.